1. Field of the Invention
The present invention relates to a specific inductor structure intended to be used, for example, in power amplifiers, transformers, etc.
2. Discussion of the Related Art
To obtain an inductor of high inductance which takes up a decreased surface area, a possibility is to form the inductor loops with metal tracks, belonging to different metallization levels, which are more or less stacked up along a privileged direction.
For certain applications, it is necessary for the inductor to further enable the conduction of strong currents. Such may be the case when the inductor is used in a power amplifier. As an example, a power amplifier made in integrated form based on MOS transistors may comprise a succession of amplification stages, among which an output amplification stage having its output corresponding to the gate of a MOS transistor and having its output corresponding to the drain of this transistor. An inductor is then generally provided between the drain and a source of a reference potential. This inductor must have a high inductance and enable the conduction of strong currents. As an example, when such an amplifier is used for the amplification of an oscillating signal at 2.4 GHz, it may be desirable for the inductor to enable the conduction of a 120-mA current at 80° C. It is in practice difficult to make, in integrated form, an inductor which at the same time takes up a decreased surface area, has a high inductance, and enables the conduction of strong currents.
An inductor generally comprises two main access terminals. For certain applications, in addition to the main access terminals, it is desirable to have an additional access terminal at the level of a specific point of the inductor for which the voltage between the additional access terminal and one of the main access terminals is opposite to the voltage between the additional access terminal and the other main access terminal. Such a point is called the inductor midpoint and the additional access terminal is called a differential access terminal. The midpoint actually corresponds to the point of the inductor for which the inductive and resistive components of the inductor between one of the main access terminals and the midpoint are identical, respectively, to the inductive and resistive components of the inductor between the midpoint and the other main access terminal.
It is desirable for the inductor to have a structure enabling to simply and systematically determine the position of the midpoint. As an example, when the inductor is formed of a single circular loop, with the two opposite ends of the loops forming the main access terminals, the midpoint corresponds to the loop point diametrically opposite to the main access terminals. However, as soon as the inductor comprises loops formed by metal tracks of several metallization levels, it may be difficult to simply and systematically determine the position of the midpoint. A difficulty is due to the fact that the materials used to form the metal tracks may be different from one metallization level to the other. As an example, for certain CMOS manufacturing processes, aluminum may be used for the last metallization level while copper may be used for the other metallization levels. Another difficulty is due to the fact that the thicknesses of the metal tracks may be different from one metallization level to another.
It is further desirable for the midpoint to be easily accessible, that is, generally, for the midpoint to be on a loop located at the level of the inductor circumference.